Question: Factor the following expression: $-9$ $x^2+$ $41$ $x$ $-20$
Answer: This expression is in the form ${A}x^2 + {B}x + {C}$ . You can factor it by grouping. First, find two values, $a$ and $b$ , so: $ \begin{eqnarray} {ab} &=& {A}{C} \\ {a} + {b} &=& {B} \end{eqnarray} $ In this case: $ \begin{eqnarray} {ab} &=& {(-9)}{(-20)} &=& 180 \\ {a} + {b} &=& & & {41} \end{eqnarray} $ In order to find ${a}$ and ${b}$ , list out the factors of $180$ and add them together. The factors that add up to ${41}$ will be your ${a}$ and ${b}$ When ${a}$ is ${5}$ and ${b}$ is ${36}$ $ \begin{eqnarray} {ab} &=& ({5})({36}) &=& 180 \\ {a} + {b} &=& {5} + {36} &=& 41 \end{eqnarray} $ Next, rewrite the expression as ${A}x^2 + {a}x + {b}x + {C}$ $ {-9}x^2 +{5}x +{36}x {-20} $ Group the terms so that there is a common factor in each group: $ ({-9}x^2 +{5}x) + ({36}x {-20}) $ Factor out the common factors: $ x(-9x + 5) - 4(-9x + 5) $ Notice how $(-9x + 5)$ has become a common factor. Factor this out to find the answer. $(-9x + 5)(x - 4)$